1. Field of Invention
The invention relates to methods and devices for assisting respiration in patients suffering from auto positive end expiratory pressure (auto-PEEP).
2. Background
Gas exchange in the human lungs occurs through a series of pressure changes that occur during the inspiration and expiration portions of the breathing cycle. During inspiration, the diaphragm descends to subsequently produce negative pressure (relative to atmospheric pressure) in the alveoli. With atmospheric pressure at the beginning of the airway (i.e., at the nose and mouth) and negative pressure in the alveoli, the pressure gradient causes air to be drawn into the airway toward the alveoli. During expiration, positive pressure (relative to atmospheric pressure) is created in the alveoli through the ascension of the diaphragm (i.e., respiratory muscles contract) and elastic recoil of the lung. The pressure gradient between the alveoli and the atmosphere causes expired breath to be expelled from the airway. During normal expiration, the pressure in the alveoli continues to decrease as breath is expired. At the end of the expiration phase, the pressure in the alveoli is equal to atmospheric pressure, and the inspiration phase begins.
In patients with lung disease, an obstruction in the airway prevents the pressure in the alveoli from decreasing to atmospheric pressure. That is, after the patient's diaphragm has fully ascended and the lung has fully recoiled, the pressure in the patient's alveoli remains above atmospheric pressure. This condition is known as auto-positive end expiratory pressure (auto-PEEP).
In patients suffering from auto-PEEP, the patient's diaphragm has to descend further (i.e., do more work) to create the negative pressure drop needed to inhale air from the atmosphere. For example, if the pressure in the alveoli at the end of the expiratory phase is 10 cm H2O above atmospheric pressure and the pressure in the alveoli needed for inspiration is −5 cm H2O relative to the pressure at the airway, the patient's diaphragm will have to descend to produce a net pressure change of −15 cm H2O in the alveoli. By contrast, if the pressure in the alveoli at the end of the expiratory phase is normal (atmospheric pressure, 0 cm H2O), the diaphragm will only have to descend to produce a net pressure change of −5 cm H2O. By requiring the diaphragm to descend further to achieve inspiration of air, the auto-PEEP resulting from lung disease causes fatigue. In extreme cases, the auto-PEEP becomes so severe (i.e., the pressure in the alveoli at the end of expiration is so high) that the diaphragm cannot descend far enough to create the negative pressure required for inspiration. Without the ability to inhale fresh air, the patient suffers respiratory distress.